PUR powder coatings, whose development received a considerable boost in the seventies, consist for the most part of organic polyhydroxyl compounds and blocked polyisocyanates (cf. e.g. DE-A 2 105 777, EP-A 23 023). Binders that are solid at room temperature represent valuable coating systems which almost achieve the flow properties of liquid PUR surface coatings and are equivalent as regards the remaining properties. The disadvantage of these systems is that, during thermal crosslinking, at least part of the blocking agents is released, causing both environmental pollution (VOC) and troublesome deposits in stove furnaces.
This disadvantage was overcome with the development of uretdione-based powder coating crosslinking agents free of blocking agents. The preparation and use of uretdione-containing powder coating crosslinking agents is described e.g. in DE-A 2 420 475 or EP-A 45 998. With these crosslinking agents, curing takes place via a thermal recleavage of the uretdione into free isocyanate groups and their subsequent reaction with a hydroxy-functional binder, this process requiring stoving temperatures above 180° C. For a long time, uretdione crosslinking agents could therefore only be used for heat-resistant substrates. Furthermore, their use was also unfavourable on economic grounds (high energy costs because of the high stoving temperatures required).
Catalyzed uretdione systems that allow crosslinking at lower temperatures have also been published recently. EP-A 803 524 described 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) as a catalyst for uretdione crosslinking at 150° C. In powder coatings conventionally used in practice, however, this catalyst causes intense yellowing. According to the teaching of WO 00/34355, uretdione powder coatings in the presence of zinc acetylacetonate as catalyst already achieve adequate crosslinking after 30 minutes at 130° C., but only with the addition of another powder coating crosslinking agent such as triglycidyl isocyanurate (TGIC).
Elimination-free powder coating crosslinking agents are described e.g. in DE-A 2 144 643 and DE-A 2 328 013. These are systems based on 2,2′-(1,4-phenylene)bis(2-oxazoline), which can be obtained by reacting aromatic dicarboxylic acids with ethanolamine to give oxazolines. A polyesterpolyamide is obtained from this structure by nucleophilic attack with polyols. It is to be regarded as a great disadvantage of this system that such surface coating systems yellow very intensely on overstoving and hence are unusable for higher-quality applications.
On the other hand, powder coating crosslinking agents based on blocked polyisocyanates which have lower crosslinking temperatures and can be cured to form optically perfect coatings without elimination of the blocking agent are not yet known.